Resonant and semi-resonant direct-current (DC) to DC power converters produce alternating currents (AC) internally that must be rectified before power is provided to a load of the power converter. The rectification may be provided by passive techniques that rely upon one or more diodes. Such passive techniques are relatively inefficient due to the voltage drop across the diode(s), and the corresponding power loss. Active rectification techniques make use of power switches, such as field-effect transistors (FETs), and are more efficient than passive techniques due to the reduced impedance of a power switch as compared to a diode. However, the power switches must be actively controlled such that they conduct current only at appropriate times.
When using power switches, henceforth termed synchronous rectification (SR) switches, for rectification, each SR switch should be disabled when the current flowing through it is zero, or as close as feasibly possible to zero. This is termed zero-current switching (ZCS), and results in minimal power loss through the SR switch(es) and a highly efficient power converter. While near-optimal ZCS is reasonably straightforward to achieve under constant-load conditions for a DC/DC power converter, complications arise as the load, and the associated current through the SR switch(es), varies.
Many prior solutions rely upon sensing the voltage across an SR switch and using this voltage to determine when to turn off the SR switch. Such solutions may require extra sensing circuitry (e.g., pins, analog-to-digital converter), as well as extra complexity to account for load variations. In order to account for load variation, some solutions use a voltage threshold that is adapted from one cycle of the DC/DC converter to the next. For example, if a particular voltage threshold leads to the SR switch being disabled after negative current has begun flowing through it for a current cycle of the DC/DC converter, then the voltage threshold is adapted (increased) so that the SR switch is disabled earlier on the next cycle. Conversely, if a particular voltage threshold leads to the SR switch being disabled while positive current is still flowing through it for a current cycle of the DC/DC converter, then the voltage threshold is adapted (decreased) so that the SR switch is disabled later on the next cycle. While such adaptation eventually leads to near-ideal ZCS of the SR switch, such adaptation takes several cycles to reach a steady-state (ideal) switch timing after a load transient. During the adaptation period, the SR switching timing is not ideal and, hence, the power efficiency is reduced.
Accordingly, there is a need for improved techniques for determining when an SR switch should be turned off within a DC/DC power converter. These techniques preferably do not require any additional circuitry beyond that which may already be available within a power converter. Furthermore, these techniques should achieve ZCS of an SR switch across changing load requirements, and the ZCS should persist during a changing load rather than only being re-achieved after some delay following a load transient.